1. Field of the Invention
The present invention relates to a method for fabricating a laminate film, and in particular, relates to a method for fabricating a display device (e.g. liquid crystal display device) using a laminate film and having improved viewing angle characteristic.
2. Description of Related Art
Liquid crystal display devices, which typify flat panel displays, have features of being lightweight, thin, and low in power consumption, compared with CRTs, and thus find applications in wide-range fields such as OA apparatus, car-mounted TV sets, car navigation systems, and monitors for video cameras.
A major problem relating to such liquid crystal display devices is that the viewing angle dependence is large. The viewing angle dependence refers to the following phenomenon, for example. When the screen of a display device is viewed from a direction tilting by an angle exceeding a certain angle range, an image that should correctly be displayed in black appears whitish, or reversal in gray scale levels is observed, causing reduction in display quality. From theses viewing directions, the viewer fails to correctly recognize the display image. When the angle range within which the viewer correctly recognizes display image is narrow, it is said that the viewing angle dependence is large.
The viewing angle dependence occurs for various reasons. These include, for example, twist orientation (i.e. helical structure) of liquid crystal molecules (the direction of a helix, the position at which liquid crystal molecules start forming a helix defined by the rubbing direction), the refractive index anisotropy of liquid crystal molecules (difference in retardation in the direction of propagation of light), the characteristics of a polarizing plate (whether or not the selectivity in the light oscillating direction is good), and the directivity of light rays from an area light source.
In general, transmission type liquid crystal display devices are designed, in consideration of the viewing angle dependence described above, so that the position at which the display can be viewed most nicely falls within the range in which the viewer normally views the display. For example, design is made so as to enhance the contrast ratio of the center area in the screen in the direction normal to the plane of the screen or in a somewhat downward direction from the viewer, compared with the surrounding areas in the screen.
By the above construction, however, the viewing angle range is still insufficient. In particular, liquid crystal display devices have large viewing angle dependence in the upward and downward directions with respect to the screen. In order to solve this problem, various methods have conventionally been proposed.
For example, Japanese Laid-Open Patent Publication No. 7-43703 discloses a liquid crystal display device in which a material is filled between a microlens array sheet and a liquid crystal display element. The material has a refractive index equal to or less than the smaller one of the refractive indices of the materials constituting the micro-lens array sheet and the liquid crystal display element.
Japanese Laid-Open Patent Publication No. 10-73808 discloses a liquid crystal display device in which a light diffusing sheet is placed on the front surface of a liquid crystal display element. The light diffusing sheet includes a first diffusion layer containing a light diffusing agent formed on a transparent member and a second diffusion layer having concave and convex portions formed on the first diffusion layer.
In both the above conventional techniques, the microlens array sheet is placed on a polarizing plate constituting the liquid crystal display element. The material having a refractive index which is different from that of the microlense array sheet is provided between the microlenses array sheet and the polarizing plate.
Japanese Laid-Open Patent Publication No. 7-120743 discloses a liquid crystal display device in which convex tip portions of a micro-lens array sheet are in close contact with the surface of a liquid crystal display element.
Japanese Laid-Open Patent Publication No. 9-127309 discloses a liquid crystal display device in which an adhesive layer is formed on convex tip portions of a micro-lens sheet. The ratio of the height A of the convex portion to the thickness B of the adhesive layer (A/B) must be more than 1 and equal to or less than 1000.
Japanese Laid-Open Patent Publication No. 9-194799 discloses a liquid crystal display device in which spacers are placed between a rough surface and an adhesive layer.
In the above conventional techniques, the convex tip portions of the microlens array sheet are partly put in contact with the liquid crystal display element via an adhesive layer, to control the proportion of the contact portion of the lens array to the non-contact portion thereof. In this way, the degrees of transmission and divergence of outgoing light are controlled, and thus the viewing angle characteristic is improved. In either case, the microlens array sheet is placed on the side of the liquid crystal display element closer to the viewer (viewer""s side), so that light outgoing from the liquid crystal display element is diffused to the side on which the microlenses are formed (lens formation directions), to attain improvement in viewing angle characteristic.
The above conventional techniques have the following problems.
In general, a pair of polarizing plates are placed on the front and rear surfaces of a liquid crystal display element for controlling the polarizing state before display of images.
The polarizing plates are made of polyvinyl alcohol (PVA) and triacetyl cellulose (TAC). PVA is impregnated with iodine, and the resultant material is drawn in one direction to align the iodine molecules, so that light polarized along the drawn direction is absorbed (or transmitted) and thus the polarizing state of incident light can be aligned in uniform.
During the above drawing, as shown in FIGS. 22A and 22B, fine waves 222 are generated on the surface of a polarizing plate 221 along an absorption axis (or a transmission axis) as the drawn direction. This is due to a miniscule variation in the thickness of the polarizing plate 221 caused by the drawing. These waves do not influence the display when they are observed only through the polarizing plate 221. However, as shown in FIG. 23B, when a light diverging element 235 such as a microlens array sheet is placed on a surface of a polarizing plate 231, in particular, when the light diverging element 235 is bonded to a polarizing plate 231 via an adhesive layer 234, waves 232 generated on the surface of the polarizing plate are magnified. As a result, the display quality greatly deteriorates.
The display quality also greatly deteriorates in the case of using a conventional double-sided adhesive tape as the adhesive layer 234 for bonding with the light diffusing element 235 and the case of using a curable resin as the adhesive layer 234. In these cases, the contact area between the light diffusing element 235 and the adhesive layer 234 is partly changed due to scars formed by hitting with foreign substances generated in the bonding process (concave and convex deformation caused by foreign substances) and deformation caused by external force (by the viewer who touches the lens surface). This partial change in the contact area causes generation of spot defects 233a and bar-shaped defects 233b as shown in FIG. 23A.
No means for solving the above problems have been mentioned in the prior art literature.
An object of the present invention is to provide a laminate film which enables an optical film to bond uniformly to a surface (e.g., surface of a display element), even if the surface has unevenness, a method for fabricating such a laminate film, a display device using such a laminate film, and a method for fabricating such a display device.
According to the first aspect of the present invention, a method for fabricating a laminate film including a transparent support having two opposing surfaces and an optical film is provided. The optical film is formed on one of the two opposing surfaces of the transparent support via an adhesive layer made of a material of which the cured state changes by application of external energy. The method includes the steps of; applying external energy to the adhesive layer; pressing the optical film against the adhesive layer to stick the optical film and the adhesive layer together; and curing the adhesive layer to a degree of hardness with which the adhesion state between the adhesive layer and the optical film is fixed while the optical film and the adhesive layer are kept stuck together.
In one embodiment of the invention, the adhesive layer is made of an ultraviolet-curable resin.
Preferably, the step of curing the adhesive layer includes the step of leaving the adhesive layer and the optical film standing while the adhesive layer and the optical film are kept stuck together. More preferably, the step of curing the adhesive layer includes the step of leaving the adhesive layer and the optical film standing while the adhesive layer and the optical film are kept stuck together so that the gel fraction of the adhesive layer is 50 wt % or more.
A surface protection film may be provided on the adhesive layer for protecting the adhesive layer, the thickness t of the surface protection film being in a range of 0.035 mmxe2x89xa6txe2x89xa60.2 mm, and the method may further includes the step of peeling off the surface protection film before the step of pressing the optical film against the adhesive layer.
A rough surface may be bonded to the other of the opposing surfaces of the transparent support via an adhesive layer. Preferably, the rough surface is a surface of a film produced by drawing. The rough surface may include a region having a roughness Rt1 satisfying Rt1 greater than 2 xcexcm when the roughness Rt1 is defined as the distance between the highest crest and the deepest trough within a range of a length evaluated.
Preferably, the transparent support has a roughness Rt2 satisfying Rt2xe2x89xa62 xcexcm when the roughness Rt2 is defined as the distance between the highest crest and the deepest trough within a range of a length evaluated.
The optical film may be a lens sheet having a plurality of lenses formed on at least one surface, and may be pressed against the adhesive layer with the surface having the plurality of lenses facing the adhesive layer. Preferably, the lens sheet is a lenticular sheet having a plurality of semi-cylindrical lenticules arranged in parallel with one another, and the lenticular sheet is pressed against the adhesive layer with a force applied in the direction of the extension of the lenticules with the surface having the plurality of lenticules facing the adhesive layer. Alternatively, the optical film may be a prism sheet having a plurality of prisms.
According to the second aspect of the present invention, a laminate film is provided. The laminate film includes: a transparent support having two opposing surfaces; an adhesive layer formed on one of the two opposing surfaces of the transparent support; and an optical film bonded to the transparent support via the adhesive layer. In the film, the adhesive layer is made of a material of which the cured state changes by application of external energy, and the transparent support has a roughness Rt satisfying Rtxe2x89xa62 xcexcm when the roughness Rt is defined as the distance between the highest crest and the deepest trough within a range of a length evaluated.
Preferably, the adhesive layer has a gel fraction of 50 wt % or more.
The optical film may be a lens sheet having a plurality of lenses formed on at least one surface, and may be pressed against the adhesive layer with the surface having the plurality of lenses facing the adhesive layer. The optical film may be a prism sheet having a plurality of prisms.
According to the third aspect of the present invention, a method for fabricating a display device including a display element and an optical film placed on the viewer""s side of the display element. The method includes the steps of: producing the display element; and bonding the optical film to the surface of the display element on the viewer""s side via an adhesive film; wherein the adhesive film includes a transparent support having a first adhesive layer formed on one of two opposing surfaces, the first adhesive layer being made of a material of which the cured state changes by application of external energy, and the step of bonding the optical film to the surface of the display element on the viewer""s side includes the steps of: applying external energy to the first adhesive layer; pressing the optical film against the first adhesive layer to stick the optical film and the first adhesive layer together; curing the first adhesive layer to a degree of hardness with which the adhesion state between the optical film and the first adhesive layer is fixed while the optical film and the first adhesive layer are kept stuck together; and after curing of the first adhesive layer, bonding the other of the two opposing surfaces of the transparent support and the display element via a second adhesive layer.
In a preferred embodiment of the invention, the display element is a liquid crystal display element including a pair of substrates, a liquid crystal material sandwiched between the pair of substrates, and optical characteristic changing means for changing the optical characteristics of incident light placed on at least the viewer""s side of the pair of substrates, and the optical film is bonded to the liquid crystal display element by bonding the optical characteristic changing means and the transparent support of the adhesive film together via the second adhesive layer.
The first adhesive layer may be made of an ultraviolet-curable resin.
The step of curing the first adhesive layer may include the step of leaving the first adhesive layer and the optical film standing while the first adhesive layer and the optical film are kept stuck together.
The step of curing the first adhesive layer may include the step of leaving the first adhesive layer and the optical film standing while the first adhesive layer and the optical film are kept stuck together so that the gel fraction of the first adhesive layer is 50 wt % or more.
Preferably, a surface protection film is provided at least on the first adhesive layer for protecting the first adhesive layer, the thickness t of the surface protection film being in a range of 0.035 mmxe2x89xa6txe2x89xa60.2 mm, and the method may further include the step of peeling off the surface protection film before the step of pressing the optical film against the first adhesive layer.
Preferably, the surface of the display element to be bonded with the second adhesive layer includes a region having a roughness Rt1 satisfying Rt1 greater than 2 xcexcm when the roughness Rt1 is defined as the distance between the highest crest and the deepest trough within a range of a length evaluated.
Preferably, the transparent member has a roughness Rt2 satisfying Rt2xe2x89xa62 xcexcm when the roughness Rt2 is defined as the distance between the highest crest and the deepest trough within a range of a length evaluated.
The optical characteristic changing means may be a polarizing plate. Alternatively, the optical characteristic changing means may be a phase plate.
The optical film may be a lens sheet having a plurality of lenses, and may be pressed against the first adhesive layer with the surface having the plurality of lenses facing the first adhesive layer.
Preferably, the lens sheet is a lenticular sheet having a plurality of semi-cylindrical lenticules arranged in parallel with one another, and the lenticular sheet is pressed against the first adhesive layer with a force applied in the direction of the extension of the lenticules with the surface having the plurality of lenticules facing the first adhesive layer. Alternatively, the optical film may be a prism sheet having a plurality of prisms.
According to the fourth aspect of the present invention, A method for fabricating a display device including a display element and a lens sheet placed on the viewer""s side of the display element is provided. The lens sheet has a plurality of lenticules arranged in parallel with one another. The method includes the steps of: producing the display element; forming an adhesive layer on the viewer""s side of the display element; placing the lens sheet so that the lens surfaces of the lenticules face the adhesive layer; and pressing the lens sheet against the adhesive layer by applying a force in the direction of the extension of the lenticules.
Hereinafter, the function of the present invention will be described.
In the method for fabricating a laminate film according to the present invention, after external energy is applied to an adhesive layer made of a material of which the cured state changes by application of external energy, an optical film (e.g., lens sheet) is pressed against the adhesive layer. This process step is carried out while the material is in B stage (intermediate cured state). The adhesive layer is then cured to a degree of hardness with which the adhesion state between the optical film and the adhesive layer no more changes, to thereby complete a laminate film. After this curing step, the material of the adhesive layer is in C stage (completely cured state) or near C stages. In such a laminate film, the adhesion state of the optical film is fixed by a transparent support via the adhesive layer. Therefore, when the laminate film is bonded to a rough surface, the shape of the rough surface is prevented from being transferred to the optical film and influencing the optical characteristics of the optical film. The effect of the present invention is especially great for an optical film having concave and convex portions formed on the surface thereof in contact with the adhesive layer in which the area of the contact region between each convex portion and the adhesive layer influences the optical characteristics of the optical film.
In particular, when the adhesive layer for bonding the optical film and the transparent support together is made of a photocurable resin, the optical film and the transparent support can be easily bonded and fixed together. This reduces generation of defects due to scars and external force.
By placing a surface protection film on the outer surface of the adhesive layer of the adhesive film and setting the thickness t of the surface protection film in the range of 0.035 mmxe2x89xa6txe2x89xa60.2 mm, the adhesive layer is prevented from deforming due to existence of foreign matters and external force before curing. As a result, bonding between the light diverging element and the transparent member is facilitated.
The method for fabricating a display device according to the present invention also has the function described above in relation with the method for fabricating a laminate film. In particular, in the case of using a liquid crystal display element as the display element, waves tend to be generated on the surface of a polarizing plate or a phase plate of the display element. When an optical film is bonded to the polarizing plate or the phase plate in an attempt to improve the characteristics of the display element, the waves of the polarizing plate or the phase plate are transferred to the optical film. According to the fabrication method of the present invention, however, the laminate film is bonded to the display element after the adhesive layer is cured to a degree that the adhesion state between the optical film and the adhesive layer no more changes. Therefore, it is possible to prevent the uneven surface of the display element from influencing the optical film, and thus prevent deterioration in display quality. In particular, when a lens sheet as the optical film is bonded to a liquid crystal display element by the fabrication method according to the present invention, the resultant liquid crystal display device can exhibit high display quality with an improved viewing angle characteristic.
By placing a surface protection film on the outer surface of the adhesive layer formed on the transparent support and setting the thickness t of the surface protection film in the range of 0.035 mmxe2x89xa6txe2x89xa60.2 mm, the adhesive layer is prevented from deforming due to existence of foreign matters and external force before curing. As a result, bonding between the optical film and the transparent support is facilitated. In particular, spot defects and bar shaped defects influence the optical performance of the light diverging element. A defect having a diameter of 0.1 mm or more will be observed as a panel defect, causing significant deterioration in display quality.
By controlling the thickness of the surface protection film, the number of spot defects and bar-shaped defects can be markedly reduced. For example, while the number of defects is 200 pieces/m2 when the thickness of the surface protection film is 0.02 mm, it can be as small as 50 pices/m2 when the thickness is 0.035 mm. For a 20-inch liquid crystal display device, for example, about 25 defects can be reduced to 10 pieces or less. The display quality therefore improves.
Although a thicker surface protection film can reduce the number of defects, it increases the cost since the material cost is higher. Therefore, the thickness is preferably 0.2 mm or less.